Assessment of Corrosion Inhibiting Efficiency of Microbes Induced Concrete

. The study present in this paper reveals the corrosion inhibiting efficiency of M25 grade concrete induced with Sporosarcina pasteurii bacteria. The accelerated corrosion induced crack method is applied on reinforced bacterial concrete which is the modified philosophy of constant voltage technique. In the current investigation, for different cover thicknesses considered, total time required for charge passed until full longitudinal crack occurs along the cover thickness due to corrosion of steel reinforcement in concrete specimens are measured using which Charge Deterioration Factors (ChDFs) are evaluated for bacterial reinforced concrete beams made with various cell concentrations of Sporosarcina pasteurii bacteria. It was established that beams made with cell concentration of 10 5 cells/ml of mixing water offers superior corrosion inhibition ability as time taken to form full length longitudinal crack is more than in other beams made with 10 3 cells/ml, 10 4 cells/ml and 10 6 cells/ml cell concentration of bacteria chosen for the study.


Introduction
Concrete is inherently offers better resistance to corrosion of rebars embedded inside it except in very harsh saline environments. If the concrete is induced with Sporosarcina pasteurii bacteria during mixing stage, will contribute to more enhanced corrosion resistance capacity due to biomineralization mechanism during microbial metabolic activity. In this paper, an attempt has been made to measure the corrosion resistance in bacterial concrete specimens with four different thicknesses of covers using the impressed constant voltage technique. It is very labor intensive and time taking experiment with a fabricated setup. It is observed that the incorporation of bacteria into concrete helps to protect concrete against the chlorideinduced corrosion of steel reinforcement by reducing its permeability, particularly for chloride-ion transportation, and increasing the impermeability of the concrete considerably.
Sporosarcina pasteurii is an alkaliphilic soil bacterium is introduced into concrete in suspension during mixing. Microbiologically Induced calcite precipitation (Biomineralization) is the process in which Sporosarcina pasteurii releases urease enzyme which hydrolyze the urea nutrient to form carbon dioxide and ammonia. Carbon dioxide reacts with calcium nutrients to produce calcium carbonate minerals which will fill up the micro and macro pores present in the concrete arresting the continuous water paths present. Ammonia will help concrete to maintain its pH so that passive layer can be formed on rebars as protective surface layer. A total of 10 beams of dimensions 150 X 150 X 500 mm are cast and tested after 28 days of normal curing. The beams have the reinforcement of 10 mm diameter HYSD bar at different effective covers ranging from 10 mm to 40 mm at all four corners of each beam.

Accelerated Corrosion Test
Due to the expansive nature of corrosion products in rebar concrete beams of M25 grade, corrosion-induced cracks are generated at the interface between the rebars and its surrounding concrete resulting in cracking and spalling of cover. To simulate this corrosion process in a typical seawater environment. The beam specimens were submerged into artificial sea water solution prepared in the E3S Web of Conferences 184, 01114 (2020) https://doi.org/10.1051/e3sconf/202018401114 ICMED 2020 laboratory as per ASTM D 1141 -1998 (Sodium chloride 24.53 g/lit, Magnesium chloride 5.2 g/lit, Sodium sulphate 4.09 g/lit, Calcium chloride 1.16 g/lit, Potassium chloride 0.695 g/lit). The specimens were corroded using an electrochemical accelerated corrosion technique that involved keeping a constant voltage of 20V through the specimens to accelerate the oxidation process in a 3.5% saline solution. When Steel rebars are corroded electrochemically eventually leads to the expansion of rebars inducing cracks in the cover of the concrete. Time taken to induce complete crack along the length of beam is computed to estimate the corrosion resistance ability of bacterial concrete. This test procedure is inspired from the standard ASTM G109. Corrosion in rebars can be artificially induced either by providing constant current through rebars or by keeping voltage constant. In this test the beams reinforced with 10 mm diameter rebars with different effective covers are casted as shown in Fig. below.

Accelerated Corrosion Test Results
Based on the experimental investigations, the test results are presented as follows-The table 1 and Fig. 6 presents time taken for formation of full crack due corrosion along the length of beams for beams with various cells of bacteria and compared with beams without bacteria. Tables 2-5 presents relative charge (rCh) and charge deterioration factors (ChDF) for various concrete cover thickness.   Cracks at concrete cover level indicates the scale of corrosion in the rebars. Occurrence of full crack at concrete cover level along the length of beam confirms 100 percent corrosion in that rebar. Time taken for occurrence of full crack at concrete cover level along the length of beam demonstrates the corrosion inhibitive ability of bacterial concrete beams of various cover thickness. From the test results 10 5 cell concentration of bacterial concrete beams fared well in protecting the steel reinforcement in concrete. At any instant of test duration, charge passed through reinforced bacterial concrete beams is less than normal reinforced beams where no bacteria is used. This indicates that flow of electrons is less in bacterial concrete beams so is the rate of corrosion. More charge passed indicates more passage of electrons so is the more probability of corrosion. Time taken for 100% passage of charge through rebars in concrete beams incorporated with optimum cell concentration of bacteria is more compared to normal beams which means bacterial concrete beams will take more time to corrode. Reinforced concrete beams admixed with optimum cell concentration took relatively more time to allow 100% charge pass through it for also cover thicknesses considered. So possible inception of corrosion in bacteria incorporated reinforced concrete beams is very low when compared to normal beams. At any instant of test duration, Charge deterioration factors will indicate the magnitude of deterioration caused due to corrosion in the beams. Charge deterioration factors will be more in normal reinforced beams. Reinforced bacterial concrete beams has less charge deterioration factors at all cover thicknesses considered. This may be due to dense microstructure of bacteria induced concrete.

Conclusions
1. The time of total charge passing till full crack failure for different effective covers considered is more for bacterial concrete than the normal concrete. This increase in total time for full crack to occur is observed in 10 5 bacterial cell concentration incorporated beams. 2. The time of total charge passing at failure in bacterial concrete is more because beam specimens did not develop any fissures or micro cracks as they did not expand when immersed in natural sea water due to calcite precipitation layer on the surface of beams. 3. The "Charge Deterioration Factors" at any given time of full crack failure for different effective covers considered is less for bacterial concrete than the normal concrete. It is concluded that that bacterial concrete will have the higher life compared to normal concrete. Bacterial concrete will have high life than the normal concrete because calcite precipitate crystals impermeable the concrete specimens and resists the harmful solutions into the concrete there by decreasing the deleterious effects they may cause.